A bomb exploded near a government building in the northern city of Aleppo yesterday, while a security crackdown prevented rallies to mark what rebels consider to be the first anniversary of the start of protests against President Bashar al-Assad.

The Syrian state news agency called the Aleppo explosion a "terrorist bombing" and said one policeman and a female civilian were killed, with 30 injured. It was the second attack in two days on regime strongholds.

Three suicide bombings in the capital, Damascus, on Saturday killed 27 people. Two of them also targeted government buildings. No one claimed responsibility for any of the attacks. The Assad regime blames the opposition, which it claims is made up of "terrorist" groups engaged in a foreign conspiracy. Aleppo and Damascus, Syria's two largest cities, have been struck by a number of suicide bombings since December. Both are critical centres of support for President Assad and have remained relatively insulated from the unrest shaking much of the country for the past year.

Mohammed Saeed, an Aleppo resident, said a car bomb exploded yesterday afternoon about 200 yards from the Political Security Directorate.

He said security forces then started shooting in the air and cordoned off the area to prevent people from approaching.

"It was a strong explosion. It shook parts of the city," Mr Saeed said, citing nearby residents. "White smoke was billowing from the area."